In physics, a wave is a disturbance or oscillation that travels through space and matter, accompanied by a
transfer of energy. Wave motion transfers energy from one point to another, often with no permanent
displacement of the particles of the medium—that is, with little or no associated mass transport. They consist,
instead, of oscillations or vibrations around almost fixed locations. Waves are described by a wave equation
which sets out how the disturbance proceeds over time. The mathematical form of this equation varies
depending on the type of wave.
Different types of waves exhibits specific characteristics. These characteristics are used to distinguish between wave types.
Orientation of particle motion relative to the direction of energy propagation is one way waves are characterized. There are
A transverse wave is a moving wave that consists of oscillations occurring perpendicular (or right
angled) to the direction of energy transfer. If a transverse wave is moving in the positive x-direction, its
oscillations are in up and down directions that lie in the y–z plane. Light is an example of a transverse
wave. For transverse waves in matter the displacement of the medium is perpendicular to the direction
of propagation of the wave. A ripple in a pond and a wave on a string are easily visualized as
Examples of transverse waves include seismic S (secondary) waves, and the motion of the electric (E)
and magnetic (M) fields in an electromagnetic plane wave, which both oscillate perpendicularly to each
other as well as to the direction of energy transfer. Therefore an electromagnetic wave consists of two
transverse waves, visible light being an example of an electromagnetic wave. See electromagnetic
spectrum for information on different types of electromagnetic waves. An oscillating string is another
example of a transverse wave; a more everyday example would be an audience wave.
Longitudinal waves, also known as "l-waves", are waves in which the displacement of the medium is in
the same direction as, or the opposite direction to, the direction of travel of the wave. Mechanical
longitudinal waves are also called compressional waves or compression waves, because they produce
compression and rarefaction when traveling through a medium. The other main type of wave is the
transverse wave, in which the displacements of the medium are at right angles to the direction of
propagation. Transverse mechanical waves are also called "t-waves" or "shear waves".
Longitudinal waves include sound waves (vibrations in pressure, particle displacement, and particle
velocity propagated in an elastic medium) and seismic P-waves (created by earthquakes and
explosions). In longitudinal waves, the displacement of the medium is parallel to the propagation of the
wave. A wave along the length of a stretched Slinky toy, where the distance between coils increases
and decreases, is a good visualization. Sound waves in air are longitudinal, pressure waves.
In physics, a surface wave is a mechanical wave that propagates along the interface between differing
media, usually two fluids with different densities. A surface wave can also be an electromagnetic wave
guided by a refractive index gradient. In radio transmission, a ground wave is a surface wave that
propagates close to the surface of the Earth.
The term "surface wave" can describe waves over an ocean, even when they are approximated by Airy
functions and are more properly called creeping waves. Examples are the waves at the surface of
water and air (ocean surface waves), or ripples in the sand at the interface with water or air. Another
example is internal waves, which can be transmitted along the interface of two water masses of